1. Field of the Invention
This invention relates generally to the inspection of a photo mask to detect contaminated particles, and more particularly, to an apparatus to calibrate the accuracy and precision of an optical mask inspection system.
2. Description of the Prior Art
As the mask and reticle designs continue to evolve in complexity and resolution, the detection of submicron contaminants on a photo mask becomes critically important. Nevertheless, the detection of these particles and the assurance of a consistent performance of high accuracy detection also become increasingly difficult. Due to the increasing integration of semiconductor circuits, submicron geometries with complicated patterns are now produced by high energy electron beams under the control of microprocessor managing the mask imaging process. Meanwhile, the high reliability requirement of the integrated circuits (ICs) demands very stringent quality assurance criteria to produce and use absolute defect-free masks. A single particle in a mask can cause manufacturing defects and render a sequence of very costly and sophisticated IC processes useless. Thus it places great importance to detect and remove any contamination or particle from the mask before the IC manufacturing process begins.
A photomask inspection system is used to perform the tasks of detecting the contaminants, determining where these particles are and measuring their sizes. The inspection is usually conducted on a patterned chrome image with a glass substrate of several hundreds square centimeters and the contaminated particles are of various sizes ranging from micrometers to sub-microns in diameter. Instead of using the high power magnifying microscope for particle detection as was practiced in the traditional manufacturing process, the technique of automatic particle detection by laser scattering is applied. Particle detection by laser scattering method is widely used for detecting particles on wafers, on liquid crystal display (LCD) substrates and on reticle/masks. Laser based systems such as QC Optics API-3000 series, Horiba PD-2000 or PD-3000 and the KLA 301/331 are applied for detection of contamination on a photomask as small as 0.5 microns. For an ideal flat surface, the reflected beam has the same angle as the incident beam relative to the flat surface and both beams lie on the same plane. However, when there are defects or contaminants on the surface, the reflected beam is scattered. By optically collecting the scattered beams and by analyzing them, the defects on a mask can be detected and measured.
The complexity of particle detection by the use of an optical inspection system is compounded by several factors. An accurate determination of the number, location, size and shape of the contaminated particles becomes a very difficult task for the following reasons:
1) The angular distribution of the laser beam scattered by the particles depends on the size and shape of the particles, the optical properties of the particles, and the wavelength and polarization of the incident light. For particles of simple shapes, such as spherical, cubic, or cylindrical shapes, there are rigorous electro-magnetic theory to obtain the analytical predictions. However, since the contaminants on a photo mask are random in shape, it is difficult to determine the sizes of the contaminants from the angular distributions.
2) The total intensity of scattering is dependent on the amount of light intercepted by the contaminants from the incident beam which is in turn determined by the particle size, or more precisely the scattering cross sections, and the intensity of the incident beam at that location. The scattering cross section, and therefore the intensity of scattering, is very sensitive to the incident angle of the inspection beam and the shape of the particle. Again, there is no way to ascertain the shape of a contaminated particle. The computation of the sizes of the detected particles are estimates at best due to this limitation.
3). As the line width of the mask becomes smaller and the pattern of a mask becomes more complex, a higher particle sensitivity is required. However, the higher the particle sensitivity the more difficult it is to differentiate between particles and pattern edges. Some new optical system incorporating polarized laser light and new position of the incident beam and detector and the differential detection methods are tested in an attempt to overcome the difficulty.
To overcome these difficulties and to assure that high quality masks are used in IC manufacture without contamination defects, the accuracy of an optical inspection system is often independently verified before an actual inspection is performed. The underlying concept is to verify the accuracy and repeatability of inspection by operating the inspection system on a flat and smooth surface with predetermined and known defects. The accuracy of the inspection system can then be determined by comparing the inspection results and the known defects previously formed on the verification surface.
The independent verification is now conducted by the use of polystyrene spheres. Polystyrene spheres whose sizes can be accurately controlled are used for testing the system performance. Diluted polystyrene liquid contained in a bottle is spreaded by an air brush onto a monitor plate. A random number of polystyrene spheres with accurately controlled size are spreaded onto the surface of a monitor plate. A particle inspection is then performed on the monitor plate. The accuracy of a mask inspection system is verified by repeated inspection of a testing plate to check the consistency of the results of particle detection between several runs of testings. The detectable particle-size threshold of a mask inspection system is determined by performing the particle detection on monitor plate with smaller and smaller polystyrene spheres until no detection of particles can be obtained from the mask inspection system.
Independent accuracy verification by use of polystyrene testing plate has several limitations.
1) There is no quantitative assurance of the number of polystyrene spheres spreaded on the testing plate. If there are inconsistency between two or more times of particle detection with an optical mask inspection system, there is no systematic method to determine which testing result is more accurate. Even if the number of spheres being detected by the optical inspection system are the same between two or more times of detections, there is still no assurance that such detections are accurate because the number of polystyrene spheres is an unknown at the outset of the tests.
2) The polystyrene spheres are randomly distributed on the surface of a testing plate. If two spheres are distributed very closely together, it may be very difficult for an optical inspection system to detect these as two independent particles because of the resolution limitation. If a polystyrene sphere is distributed very close to the edge or the corner of a pattern, the accuracy of the independent verification is hampered because the optical diffraction pattern from the edge or the corner. The diffraction patterns from the edge or corner of a pattern may often obscure the reflecting beam from the polystyrene spheres and make the determination of the location and number of particles on the monitor plate quite complicated.
3) The polystyrene spheres spreaded on the test plate are not securely disposed on the surface. Only a very weak electro-static force binds those spheres to the testing plate surface. Random motions of these polystyrene spheres may occur which may change the detection results between repeated particle detection by a mask inspection system. Repeated use of a polystyrene testing plate is therefore not reliable unless the plate is handled very carefully to assure no random motion of the polystyrene spheres has occurred between tests. Storage of a polystyrene testing plate for later use is extremely difficult because of this limitation and also because the testing plate cannot be cleaned or decontaminated before a later use to assure the quality of testing.
The state-of-the-art optical mask inspection system is therefore limited by these difficulties. Due to these limitations, the accuracy of an optical mask inspection system cannot be independently verified with assurance of ascertainable accuracy. Because of the critical importance in assuring defect-free photo masks in manufacturing the integrated circuits, there is still a great need to overcome these aforementioned limitations.